The present invention relates to a stencil printing method for printing a color image on a sheet by conveying the paper sheet only once and a stencil printer for practicing the same.
It is a common practice with a stencil printer to arrange a plurality of print drums each being assigned to a particular color in a direction in which a paper is conveyed. While a paper sheet is conveyed only once, ink images of different colors are sequentially transferred from the print drums to the paper sheet one above the other, forming a composite color image. Such a single pass, color printing system is far more efficient than a system that requires a print drum to be replaced color by color and repeatedly feeds the same paper sheet. This kind of system, however, has some problems left unsolved due to a short interval between consecutive printing positions. Specifically, so-called retransfer repeatedly occurs in this kind of system on the second and successive paper sheets. A density difference between the first and second colors sequentially increases and degrades image equality.
Further, an increase in the density of an ink image of the first color directly translates into an increase in the adhering force of ink forming the image. The ink is therefore apt to prevent a paper sheet from being separated from a print drum assigned to the second color and cause it to roll up together with the print drum being rotated.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 1-290489, 3-55276, 6-32038, 7-17121, 10-297073, 10-305649 and 11-34467, and GB 2,277,904A.
It is therefore an object of the present invention to provide a stencil printing method capable of obviating a density difference and the roll-up of a paper sheet ascribable to retransfer and a stencil printer for practicing the same.
In accordance with the present invention, in a stencil printing method for printing a color image on a paper sheet by passing a paper sheet only once through a stencil printer in which a plurality of print drums each having a respective master wrapped therearound are spaced from each other in a direction of paper conveyance, ink is transferred from an upstream print drum, in the above direction, to the paper sheet in a smaller amount than ink transferred from a downstream print drum to the same paper sheet such that the former ink is equal in amount to the latter ink at the end of a printing cycle.
Also, in accordance with the present invention, a stencil printer for printing a color image on a paper sheet by passing the paper sheet only once therethrough includes a plurality of print drums each having a respective master wrapped therearound and spaced from each other in a direction of paper conveyance. An ink transfer adjusting device causes ink to be transferred from an upstream print drum, in the above direction, to the paper sheet in a smaller amount than ink transferred from a downstream print drum to the same paper sheet such that the former ink is equal in amount to the latter ink at the end of a printing cycle.